44 PRACTICAL ORGANIC CHEMISTRY water running back when the aspirator is stopped. Before stopping the pump, close the glass tap. Turn off the water, and then lift the tap out of its socket for a moment to equalise the pressure. Use a porcelain funnel and filter flask, different sizes of which are shown in Fig. 36. The bottom of the funnel is covered with a disc of filter paper. After filtering, wash three or four times with a little cold water, press the precipitate well down and let it drain. Remove the precipitate and spread it on a piece of porous plate, and place it in a vacuum-desiccator over sulphuric acid. There are several use- ful forms of vacuum- desiccator, two of which are represented in Fig. 37. The ground rims are greased with vase- line or a mixture of bees-wax and vaseline, and the air is exhausted by attaching the tube of the water-pump to FIG. 35. the glass tap of the desiccator. If the substance is left overnight in the desiccator it will be dry by the next day. The silver salt should be protected as far as possible from the light. When the precipitate is thoroughly dry, weigh about 0-3 gram into a weighed porcelain crucible. Cover with the lid and heat, at first gently, over a small flame. When the first reaction is over, heat the crucible for a few minutes to a dull red heat, and then allow it to cool in a desic- cator. The silver salt will be completely decomposed and leave a dull white residue of silver. The crucible is now weighed and the weight of silver determined.